C++ Notes: Tuple

I have known tuple​ of Python for a long time, but I just found that C++ also provides $tuple$.

1. The tuple​ Type

• A tuple is a template that is similar to a pair.
• A tuple has members whose types vary from one tuple type to another, but a tuple can have any number of members.

tuple<type1, type2, type3> t1;
t1 = make_tuple("John Wick", "Male", 45);
tuple<size_t, int, char, bool> t2(1, 2, 'a', true); // ok
tuple<double, double, double> threeD{1, 2, 3};      // ok

2. Accessing the Members of a $tuple$

Access the members of a $tuple$ through a library function template named get. Pass a tupleobject to get, and it can return a reference to the specified member:

auto x = get<0>(threeD);  // returns the first member of threeD
auto y = get<1>(threeD);  // returns the second member of threeD

To know the number of members in a tuple and the type of a specific member, we use two auxiliary class templates, tuple_size and tuple_element.

size_t sz = tuple_size<threeD>::value;  // returns 3
typedef decltype(threeD) trans;         // trans is the type of threeD
tuple_element<2, trans>::type z = get<int>(threeD);  // z is a double

3. Relational and Equality Operators

We can compare two tuples only if they have the same number of members, otherwise the program will report errors.

4. Using a tuple to Return Multiple Values

A common use of tuple is to return multiple values from a function. Just like Python, we can group several values together in the end of a function and return it as a tuple.

5. tuple + Structured Binding

Structured Binding is a C++17 feature which provides a new way to allow a single definition to define multiple variables with different types. Like a reference, a structured binding is an alias to an existing object. Unlike a reference, the type of a structured binding does not have to be a reference type. We can use a structured binding with an array, data members of a class, or a tuple.

auto [a, b, c] = threeD;  // a == 1.0, b == 2.0, c == 3.0

Be careful! Structured binding only works if the structure is known at compile time, so it doesn’t work for the vector.

References

• Stanley B. Lippman. C++ Primer (5th Edition)

• https://en.cppreference.com/w/cpp/language/structured_binding